Reports: DNI349646-DNI3: New Catalysis with Nickel Pincer Complexes

For
the second year of the project, we have devoted much of our efforts to improving
the catalytic performance of nickel
bis(phosphinite)
pincer complexes in the reduction of CO2 to methanol derivatives. We have compared the catalytic activity of a
series of nickel pincer hydride complexes bearing different sizes of alkyl
groups on the phosphorous donor atoms.
For the reaction of CO2 with catecholborane, the most
efficient catalyst involves the complex with the most sterically hindered
hydride ligand. We have investigated
other boranes and silanes as the reducing
reagents. 9-borabicyclo[3.3.1]nonane behaves similarly as HBcat, while the reaction of pinacolborane
stops at the formate stage rather than the methoxy
stage. In contrast, phenylsilane
is not a viable reducing reagent for nickel-catalyzed hydrosilylation of CO2.

We have also explored other catalytic applications
of nickel bis(phosphinite) pincer complexes. We have shown that [2,6-(Ph2PO)2C6H3]NiClcatalyzes cross-coupling of aryl iodides and
aryl thiols. The optimal catalytic
conditions involve 1 mol% of the nickel catalyst and 2 equiv of KOH (with
respect to aryl thiols) in DMF at 80 °C, and tolerate a variety of
functional groups in the substrates. Mechanistic
studies have suggested that the pincer ligand framework in the nickel complex
is destroyed by KOH via the cleavage of P-O bonds to release Ph2POK,
and further decomposition leads to Ph3P and other
phosphorus-containing products. The
cross-coupling reactions are more effectively catalyzed by Ni(COD)2/Ph2P(O)H.

We have extended our studies
beyond the chemistry of nickel. As our
initial efforts on iron catalysis, we have synthesized iron bis(phosphinite) pincer
hydride complexes via the C-H bond activation of resorcinol-derived ligands
1,3-(R2PO)2C6H4 (R = iPr and Ph) with Fe(PMe3)4.
These new iron POCOP-pincer hydride
complexes catalyze the hydrosilylation of aldehydes and ketones with different
functional groups. Isotopic labeling
experiments rule out the hydride ligand being directly involved in the
reduction. The hydrosilylation reactions
are more likely to proceed via the activation of silanes or carbonyl substrates
after ligand dissociation from the iron center.

This ACS PRF grant has made a
significant impact on the career of PI as well as the students that were
supported by the grant. The grant
provided the seed money that eventually led to a successful NSF career
award. Two undergraduate students
working on the project have co-authored a paper and one of them is currently a
graduate student in a chemistry PhD program.